Wireless transmission method, and wireless transmitter and wireless receiver

ABSTRACT

A wireless transmitter controls the number of transmission beams to be formed for transmitting a data stream depending on the number of data streams to be transmitted, and a wireless receiver selectively receives any one or more of the transmission beam from the transmission beams. In this manner, by changing the number of transmission beams (the original number of beams selectable on the receiving side) to be formed depending on the number of transmitting data streams, high throughput characteristics by a low interbeam correlation at the time of the multistream transmission and a large directional gain at the time of a single stream may be achieved.

The present invention is a continuation of International ApplicationNumber PCT/JP2006/301776 filed Feb. 2, 2006, the entirety of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a wireless transmission method, and awireless transmitter and a wireless receiver, and for example, relatesto a technique for use in a multiple-input and multiple-output wirelesstransmission technique for performing signal transmission by using aplurality of transmitting and receiving antennas in a wirelesscommunication system such as a mobile-phone and a wireless access.

BACKGROUND ART

Recently, a MIMO (Multiple-Input Multiple-Output) has drawn attention asthe technique for enabling a high-capacity (high-speed) datacommunication by effectively using a frequency band. The MIMO is thetechnique to transmit separate data streams from a plurality of antennasof a transmitter by using a plurality of antennas in both of thetransmission and the reception, that is to say, by using the transmitterhaving a plurality of antennas and the receiver having a plurality ofantennas, and individually separate a plurality of transmission signals(data streams) mixed on a transmission path from the signal received byeach receiving antenna of the receiver by using a transmission path(channel) estimate value, thereby improving a transmission rate withoutrequiring an enlargement of the frequency band.

FIG. 8 illustrates a configuration example of the previous MIMOtransmission system. The system illustrated in FIG. 8 corresponds to asystem shown in FIG. 1 of the Non-Patent Document 1 to be describedlater, and is provided with a MIMO transmitter 100 and a MIMO receiver200; focusing on substantial parts thereof, the MIMO transmitter 100 isprovided with a user selector 101, a channel coder/modulator 102, a beamselector 103, a multibeam former 104, a scheduler 105 and a plurality oftransmitting antennas 106, and the MIMO receiver 200 is provided with aplurality of receiving antennas 201, a MIMO/SIMO demodulator 202, achannel decoder 203, a transmission beam measure 204 and a transmissionbeam/stream determiner 205.

Also, in the MIMO transmitter 100, in the user selector 101, under thecontrol of the scheduler 105, one or more user data stream to betransferred is selected from a plurality of series of user data streamsand is input to the channel coder/modulator 102, and in the channelcoder/modulator 102, under the control of the scheduler 105, a requirederror correction coding such as a turbo coding is performed with aspecified coding ratio, and after that, obtained bit series is mapped toa specified modulation scheme, for example, a symbol having a signalpoint (signal of the data channel) such as QPSK (Quadrature Phase ShiftKeying) and 16QAM (Quadrature Amplitude Modulation) and is modulated.Meanwhile, in the channel coder/modulator 102, in addition to the datachannel signal, the signal of the pilot channel (pilot symbol) used forchannel estimation and the signal of the control channel (controlsymbol) transmitting the control information may be multiplexed.

The modulated data thus-obtained is input to the beam selector 103, andin the beam selector 103, under the control of the scheduler 105, thebeam used for transmitting the modulated data is selected from aplurality of fixed beams (multibeam) formed by the multibeam former 104by just the number of streams to be transmitted and the modulated datais transmitted from the transmitting antenna 106 by the selected beam.

For example, assuming that the number of transmitting antennas 106 isfour and the number of fixed beams capable of being formed by themultibeam former 104 is four at the maximum, when the number of streamsto be transmitted is four, all of the four beams are selected, and in acase of two streams, two beams are selected out of four beams, and in acase of one stream, one stream is selected out of four beams.

On the other hand, in the MIMO receiver 200, a wireless signaltransmitted from the transmitting antenna 106 of the MIMO transmitter100 is received by each receiving antenna 201, and MIMO demodulated orSIMO (Single-Input Multi-Output) demodulated by the MIMO/SIMOdemodulator 202, and the user data stream is generated. That is to say,in the MIMO/SIMO demodulator 202, the user data streams multiplexed foreach of the transmitting antennas 106 are separated by a method of usingan inversion matrix of a channel correlation matrix and a method ofusing an MLD (Maximum Likelihood Detection) algorithm, based on achannel estimate value (channel matrix) obtained by a correlationcalculation of the received pilot symbol and the pilot replica, and thedemodulated data is generated.

The obtained demodulated data is input to the channel decoder 203, andan error correction decoding such as a turbo decoding is performed inthe channel decoder 203, and decoded data of the user stream received bythe data channel may be obtained.

Meanwhile, each signal received at the receiving antenna 201 is alsoinput to the transmission beam measure 204, and a CQI (Channel QualityIndicator) value, which is an index of reception quality, is measuredbased on the received pilot symbol in the transmission beam measure 204,and one or more beam of which reception quality is the best isdetermined (selected) based on the obtained CQI value in thetransmission beam/stream determiner 205. Then, information including thedetermined number of beams, corresponding CQI value and the beam ID isgenerated as feedback information to the MIMO transmitter 100 and istransmitted to the MIMO transmitter 100.

The above-described feedback information is finally reported to thescheduler 105 of the MIMO transmitter 100, and thereby, the scheduler105 controls the user selector 101, the channel coder/modulator 102 andthe beam selector 103 so as to transmit the transmission user datastream as described above by the beam of the number of beams (beam ID)determined (selected) in the MIMO receiver 200 (transmission beam/streamdeterminer 205) and by the coding ratio and modulation scheme dependingon the reported CQI value.

Meanwhile, as disclosed in the Patent Document 1 to be described later,in the closed loop type MIMO transmission scheme, which performspre-coding on the transmitting side, it is also required to send backthe information of the channel matrix or the received weight (weightingcoefficient of the multibeam) obtained on the receiving side, as thefeedback information to the transmitting side.

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2005-311902-   Non-Patent Document 1: 3GPP TSG RAN WGI meeting #43 (R1-051438),    “Multi-beam MIMO for EUTRA Downlink”, Fujitsu, November 2005

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

For improving the transmission rate by the MIMO multiplex method, (1) ahigh SNR (Signal to Noise Ratio) and (2) a low interantenna correlation(or a low interbeam correlation) are required. In a case in which thecondition is not satisfied, throughput characteristics by the MIMOmultiplex are significantly deteriorated, so that it is advantageous touse the MIMO diversity or directional beam transmission for the sake ofthe throughput of the entire system.

Herein, in the above-described previous technique, since the number ofbeams to be formed is constant irrespective of the number oftransmission streams (for example, fixed to the maximum value of thenumber of beams capable of being formed) [in other words, a beamdivergence (directional intensity) for one beam is constant], the effectby the MIMO multiplex is not obtained depending on the beam to beselected, so that the throughput characteristics may be deteriorated.

For example, if the beams having a high correlation therebetween (forexample, adjacent beams) are selected on the transmitting side by thefeedback information from the receiving side, the separation and thedemodulation processing capability of the user data streams on thereceiving side are deteriorated. Therefore, if the beams having a lowcorrelation therebetween are selected, deterioration in such aseparation and demodulation processing capability may be suppressed;however, this is more deteriorated than the reception quality by thebeam, which is supposed to be selected as the one of which receptionquality (directional gain) is excellent for the receiving side due tothe directionality of the beam.

Herein, as disclosed in the Patent Document 1, although it is possibleto adjust the selected interbeam correlation and the beam directionalityto ease the deterioration in the separation and the demodulationprocessing capability and the deterioration in the reception quality byfeed backing the channel matrix and the reception weight used on thereceiving side as the feedback information to the transmitting side, thefeedback information amount is increased and the arithmetic processingfor adjustment is required.

The present invention is made in view of such a problem, and an objectthereof is to combine the high throughput characteristics by the lowinterbeam correlation and the high directional gain to obtain theexcellent reception characteristic without increasing the feedbackinformation amount in the MIMO transmission.

Means for Solving the Problem

In order to achieve the above-described object, the present inventionuses a wireless transmission method, and a wireless transmitter and awireless receiver to be described below. That is to say,

(1) As a generic aspect, there provided is the wireless transmissionmethod capable of transmitting a data stream between a wirelesstransmitter having a plurality of transmitting antennas and a wirelessreceiver having a plurality of receiving antennas by a multibeam, themethod including: controlling the number of transmission beams to beformed for transmitting the data stream depending on the number of datastreams to be transmitted at the wireless transmitter, and selectivelyreceiving any one or more transmission beam from the transmission beamsby the wireless receiver.

(2) Herein, the wireless transmitter may control to increase the numberof transmission beams in proportion as the number of transmission datastreams is smaller.

(3) Also, the wireless receiver may selectively receive two or more ofthe transmission beams having a low correlation therebetween, when thenumber of transmission data streams is two or larger.

(4) Further, the wireless receiver may selectively receive nonadjacenttransmission beams as the transmission beams having a low correlationtherebetween.

(5) In addition, the wireless transmitter may multiplex a pilot signalfor each of the transmitting antennas to perform beam transmission by afixed weighting coefficient, and the wireless receiver may measure alevel of the transmission beam based on the pilot signal and theweighting coefficient, determine the number of transmission data streamsand the transmission beam to be received based on the measured level,and report information regarding the number of transmission data streamsand the transmission beam, which are determined, to the wirelesstransmitter, and the wireless transmitter may control the number oftransmission beams based on the information reported from the wirelessreceiver.

(6) Further, the wireless transmitter may multiplex a pilot signal foreach of the transmission beams to perform beam transmission by a fixedweighting coefficient, and the wireless receiver may measure a level ofthe transmission beam based on the pilot signal, determine the number oftransmission data streams and the transmission beam to be received basedon the measured level, and report information regarding the number oftransmission data streams and the transmission beam, which aredetermined, to the wireless transmitter, and the wireless transmittermay control the number of transmission beams based on the informationreported from the wireless receiver.

(7) Alternatively, the wireless transmitter may multiplex a pilot signalfor each of the transmitting antennas to perform beam transmission by avariable weighting coefficient, and broadcast information regarding theweighting coefficient and information regarding the number oftransmission beams to the wireless receiver, and the wireless receivermay measure a level of the transmission beam based on the pilot signaland the information regarding the weighting coefficient broadcasted fromthe wireless transmitter, determine the number of transmission datastreams and the transmission beam to be received based on the measuredlevel and the information regarding the number of transmission beamsbroadcasted from the wireless transmitter, and report informationregarding the number of transmission data streams and the transmissionbeam, which are determined, to the wireless transmitter, and thewireless transmitter may control the number of transmission beams basedon the information reported from the wireless receiver.

(8) As another generic aspect, there provided is a wireless transmitterwhich is capable of transmitting a data stream to a wireless receiverhaving a plurality of receiving antennas by a multibeam, and is providedwith a plurality of transmitting antennas, and number-of-transmissionbeams control means operable to control the number of transmission beamsto be formed for transmitting the data stream depending on the number ofdata streams to be transmitted from the transmitting antennas.

(9) Herein, the number-of-transmission beams control means may controlto increase the number of transmission beams in proportion as the numberof transmission data streams is smaller.

(10) Also, this transmitter may be further provided with first pilotmultiplex means operable to multiplex a pilot signal for each of thetransmitting antennas; a first beam former operable to perform beamtransmission by a fixed weighting coefficient, and first reportedinformation receiving means to receive information regarding the numberof transmission data streams and the transmission beam, determined basedon a level measurement result measured for the transmission beam basedon the pilot signal and the weighting coefficient in the wirelessreceiver and reported from the wireless receiver, wherein thenumber-of-transmission beams control means may control the number oftransmission beams based on the information received by the firstreported information receiving means.

(11) Further, this transmitter may be further provided with second pilotmultiplex means operable to multiplex a pilot signal for each of thetransmission beams, a first beam former operable to perform beamtransmission by a fixed weighting coefficient, and second reportedinformation receiving means to receive information regarding the numberof transmission data streams and the transmission beam, determined basedon a level measurement result measured for the transmission beam basedon the pilot signal in the wireless receiver and reported from thewireless receiver, wherein the number-of-transmission beams controlmeans may control the number of transmission beams based on theinformation received by the second reported information receiving means.

(12) Also, this transmitter may be further provided with first pilotmultiplex means operable to multiplex a pilot signal for each of thetransmitting antennas, a second beam former operable to perform beamtransmission by a variable weighting coefficient, broadcasting meansoperable to broadcast information regarding the weighting coefficientand information regarding the number of transmission beams to thewireless receiver, and third reported information receiving means toreceive information regarding the number of transmission data streamsand transmission beam, determined based on a level measurement resultmeasured for the transmission beam based on the pilot signal and theinformation regarding the weighting coefficient broadcasted by thebroadcasting means and the information regarding the number oftransmission beams broadcasted by the broadcasting means, and reportedfrom the wireless receiver, wherein the number-of-transmission beamscontrol means may control the number of transmission beams based on theinformation received by the third reported information receiving means.

(13) As still another generic aspect, there provided is a wirelessreceiver which is capable of receiving a data stream from a wirelesstransmitter having a plurality of transmitting antennas by a multibeam,and is provided with a plurality of receiving antennas, and beamselective reception control means operable to selectively receive anyone or more transmission beam through the receiving antennas from thetransmission beams of which number of transmission beams to be formedfor transmitting the data stream is controlled depending on the numberof data streams to be transmitted by the wireless transmitter.

(14) Herein, the beam selective reception control means may selectivelyreceive two or more of the transmission beams having a low correlationtherebetween, when the number of transmission data streams is two orlarger.

(15) Also, the beam selective reception control means may selectivelyreceive nonadjacent transmission beams as the transmission beams havinga low correlation therebetween.

(16) Further, the wireless transmitter may multiplex a pilot signal foreach of the transmitting antennas to perform beam transmission by afixed weighting coefficient, and the beam selective reception controlmeans may be provided with a first level measuring section to measure alevel of the transmission beam based on the pilot signal and theweighting coefficient, a first determining section operable to determinethe number of transmitting data streams and the transmission beam to bereceived based on the level measured by the first level measuringsection, and a first report section to report information regarding thenumber of transmission data streams and the transmission beam determinedby the first determining section as control information of the number oftransmission beams in the wireless transmitter.

(17) In addition, the wireless transmitter may multiplex a pilot signalfor each of the transmitting beams to perform beam transmission by afixed weighting coefficient, and the beam selective reception controlmeans may be provided with a second level measuring section to measure alevel of the transmission beam based on the pilot signal, a seconddetermining section operable to determine the number of transmissiondata streams and the transmission beam to be received based on the levelmeasured by the second level measuring section, and a second reportsection to report information regarding the number of transmission datastreams and the transmission beam determined by the second determiningsection as control information of the number of transmission beams inthe wireless transmitter.

(18) Further, the wireless transmitter may multiplex a pilot signal foreach of the transmitting antennas to perform beam transmission by avariable weighting coefficient, and broadcast the information regardingthe weighting coefficient and the information regarding the number oftransmission beam to the wireless receiver, and the beam selectivereception control means may be provided with a third level measuringsection to measure a level of the transmission beam based on the pilotsignal and the information regarding the weighting coefficientbroadcasted from the wireless transmitter, a third determining sectionoperable to determine the number of transmission data streams and thetransmission beam to be received based on the level measured by thethird level measuring section and the information regarding the numberof transmission beams broadcasted from the wireless transmitter, and athird report section to report information regarding the number oftransmission data streams and the transmission beam determined by thethird determining section as control information of the number oftransmission beams in the wireless transmitter.

Effect of the Invention

According to the above-described aspects, at least any of followingeffect or advantage may be obtained.

(1) In the transmitter, the number of transmission beams (the number ofselectable beams of a transmission source) to be formed depending on thenumber of data streams to be transmitted is controlled (changed), sothat it becomes possible to obtain good throughput characteristics(reception characteristics) without increasing the feedback informationamount from the receiver to the transmitter.

(2) For example, by increasing the number of transmission beams inproportion as the number of transmission data streams is smaller, thenumber of selectable transmission source beams increases, so that itbecomes possible to obtain the high directional gain.

(3) Also, in a case in which the number of transmission data streams istwo or larger, by selectively receiving two or more transmission beamshaving a low correlation therebetween (for example, nonadjacent), itbecomes possible to avoid deterioration of data stream separationcapability on the receiving side and obtain high throughputcharacteristics due to the low interbeam correlation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for illustrating an overview of an embodiment;

FIG. 2 is a block diagram illustrating a configuration of a MIMOtransmission system according to a first embodiment;

FIG. 3 is a block diagram illustrating a configuration focusing on atransmission beam ID/stream determiner and a known selectable beammemory in FIG. 2;

FIG. 4 is a flowchart for illustrating an operation (beam selectionmethod) of the transmission beam ID/stream determiner shown in FIG. 3;

FIG. 5 is a schematic diagram illustrating one example of selectablebeams for illustrating an operation of the MIMO transmission systemshown in FIG. 2;

FIG. 6 is a block diagram illustrating a configuration of the MIMOtransmission system according to a second embodiment;

FIG. 7 is a block diagram illustrating a configuration of the MIMOtransmission system according to a third embodiment; and

FIG. 8 is a block diagram illustrating a configuration of the previousMIMO transmission system.

EXPLANATIONS OF REFERENCE NUMERALS

-   -   1 MIMO transmitter    -   11 user selector    -   12 channel coder/modulator    -   13 beam selector    -   14 multibeam former    -   15 scheduler (beam controller) (number-of-transmission beam        control means, first, second, third reported information        receiving means)    -   16-1 to 16-n transmitting antennas    -   17 element pilot multiplexer (first pilot multiplex means)    -   17-1 to 17-n adders (multiplex circuit)    -   17 a beam pilot multiplexer (second pilot multiplex means)    -   17 a-1 to 17 a-n adders (multiplex circuits)    -   18 broadcast information adder (broadcasting means)    -   19 a weight generator    -   19 b selectable beam information generator    -   2 MIMO receiver    -   20 beam selective reception control means    -   21-1 to 21-M receiving antennas    -   22 MIMO/SIMO demodulator    -   23 decoder (channel decoder)    -   24 transmission beam measure (first, second, third level        measuring section)    -   25 known pilot memory    -   26 known transmission weight memory    -   27 transmission beam ID/stream determiner (first, second, third        determining section, first, second, third report section)    -   271 ranking level comparator    -   28 known selectable beam memory    -   281 comparative beam ID table    -   29 broadcast information extractor    -   30-0 to 30-9 beams

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention is described withreference to the drawings.

[A] Overview

First, an overview of the embodiment to be described below is describedby using FIG. 1. In FIG. 1, reference numerals 1 and 2 represent a MIMOtransmitter provided with a plurality of (herein, four) transmittingantennas and a MIMO receiver provided with a plurality of receivingantennas, respectively, and it is configured such that wireless MIMOtransmission is performed between the MIMO transmitter 1 and the MIMOreceiver 2. The MIMO transmitter 1 is applicable, for example, as a basestation device of a mobile wireless communication system, and the MIMOreceiver 2 is applicable as a mobile station device (UE: User Equipment)of the system. Therefore, in a following description, the MIMOtransmitter 1 is also represented as a base station device 1 or a basestation 1, and the MIMO receiver 2 is also represented as a mobilestation device 2 or a mobile station 2. In addition, a detailedspecification conforms to a Table A1 in the Non-Patent Document 1, forexample.

Also, in this example, the base station device 1 is configured to beable to change (control) the number of beams to be formed (beamforming)depending on the number of user data streams to be sent (transmitted)(hereinafter, also simply referred to as “transmission stream”), and themobile station device 2 is configured to be able to selectively receiveany one or more beam from a multibeam having the above-described numberof beams.

For example, in the base station 1, when the number of transmissionstreams is not large, or in the case of a single stream at the minimum,the mobile station 2 selectively receives, for example, the beam ofwhich reception level is the maximum out of more beams formed by thebase station 1. Also, as the number of transmission streams increases, acombination of selectable beams is limited. When the number oftransmission streams is large, in a case of transmitting the multistreamof up to the number of transmitting antennas, the beam by elementtransmission (also construable that there is only one selectable numberof beams) is selectively received.

Meanwhile, in FIG. 1, cases in which (1) the number of transmissionstreams is four, (2) the number thereof is two and (3) the numberthereof is one in the base station 1 are shown, respectively, and it isillustrated that in the case of (1), the base station 1 forms one beamby each of the transmitting antennas and element transmits four streamsby the beam, and the mobile station 2 directly (without selecting thebeam) receives the signal, which is element transmitted by one beam, inthe case of (2), the base station 1 forms four beams and transmits twostreams by the four beams, and the mobile station 2 selectivelyreceives, for example, two beams having low interbeam correlationtherebetween out of the four beams, and in the case of (3), the basestation 1 forms eight beams and transmits one stream by the eight beams,and the mobile station 2 selectively receives one beam out of the eightbeams, respectively.

In this manner, it becomes possible to obtain excellent throughputcharacteristics and a directional gain, by making it possible to obtainthe gain as large as possible at the time of one stream and by selectingthe beams such that the interbeam correlation is low at the time ofmultistream, by changing the original number of beams selectabledepending on the number of transmission streams, that is to say, thenumber of transmission beams to be formed (beamforming).

Hereinafter, a specific example is described in detail.

[B] Description of First Embodiment

FIG. 2 is a block diagram illustrating a configuration of the MIMOtransmission system according to a first embodiment, and the MIMOtransmission system shown in FIG. 2 is provided with the MIMOtransmitter 1 and the MIMO receiver 2; focusing on substantial partsthereof, the MIMO transmitter 1 is provided with a user selector 11, achannel coder/modulator 12, a beam selector 13, a multibeam former 14, ascheduler (beam controller) 15, a plurality of transmitting antennas16-1 to 16-n (n is an integer of 2 or larger) and an element pilotmultiplexer 17, and the MIMO receiver 2 is provided with one or aplurality of receiving antennas 21-1 to 21-M (M is an integer of 1 orlarger and possibly M=n), a MIMO/SIMO demodulator 22, a decoder (channeldecoder) 23, a transmission beam measure 24, a known pilot memory 25, aknown transmission weight memory 26, a transmission beam ID/streamdeterminer 27 and a known selectable beam memory 28. In what follows,the MIMO transmitter 1 may be referred to simply as “transmitter 1” or“transmitting side 1” and the MIMO receiver 2 may be referred to simplyas “receiver 2” or “receiving side 2”.

Here, in the MIMO transmitter 1, the user selector 11 is operable toselect one or more user data stream to be transmitted from a pluralityof series of user data streams under the control of the scheduler 15,and the channel coder/modulator 12 is operable to perform a requirederror correction coding such as a turbo coding with a specified codingratio under the control of the scheduler 15, and mapping obtained bitseries to a specified modulation scheme, for example, a symbol having asignal point (signal of data channel) such as QPSK (Quadrature PhaseShift Keying) or 16QAM (Quadrature Amplitude Modulation), therebymodulating the same.

The beam selector 13 is operable to select one or a plurality of beamused for transmitting the transmission stream (user data) coded andmodulated by the channel coder/modulator 12, from a plurality of beams(multibeam) formed by the multibeam former 14, under the control of thescheduler (beam controller) 15, in greater detail, depending on feedbackinformation (information regarding a transmission beam ID and thetransmission stream) from the receiving side 2. The transmission beam ID(identification information) is uniquely defined (set) based on atransmission weight matrix W used in the multibeam former 14 to bedescribed below (same as above).

The multibeam former (first beam former) 14 is operable to form themultibeam for transmitting the transmission stream based on apredetermined transmission weight matrix (weighting coefficient) W. Inthis example, the transmission weight matrix W is fixed.

The element pilot multiplexer (first pilot multiplex means) 17 isoperable to multiplex an orthogonal pilot signal (symbol) pi for each ofthe transmitting antennas 16-i (i=1 to n) by adders (multiplex circuits)17-1 to 17-n of the transmitting antennas 16-1 to 16-n, respectively,and thereby, the orthogonal pilot signal pi is transmitted for each ofthe transmitting antennas 16-i (element).

The beam controller (scheduler; number-of-transmission beam controlmeans) 15 is operable to control the number of transmission beams formedfor transmitting the transmission stream depending on the number oftransmission streams by controlling beam selection in theabove-described beam selector 13, and in this example, it is configuredto receive the information regarding the beam ID and the number ofstreams determined (selected) by the transmission beam ID/streamdeterminer 27 on the receiving side 2 as feedback information to controlthe number of transmission streams and the beam (the number oftransmission beams to be formed) used for transmitting the transmissionstream based on the feedback information.

On the other hand, in the receiver 2, the receiving antennas 21-j (j=1to M) receives the beam transmitted from each of the transmittingantennas 16-i of the transmitter 1, and the MIMO/SIMO demodulator 22 isoperable to MIMO-demodulating or SIMO-demodulating the signal receivedby each of the receiving antennas 21-j, and the demodulated data isgenerated by separating the user data streams, which are multiplexed foreach of the transmitting antennas 16-i, by a method of using aninversion matrix of a channel correlation matrix and a method of usingan MLD algorithm, based on a channel estimate value (channel matrix)obtained by a correlation operation of the pilot signal pi and the pilotreplica multiplexed on a received signal.

The decoder 23 decodes the user data stream obtained by theabove-described MIMO/SIMO demodulator 22 by a decoding schemecorresponding to the coding scheme in the transmitting side 1.

The known pilot memory 25 stores a replica signal (pilot replica) of thepilot signal pi in advance, the known transmission weight memory 26 isfor storing information of the transmission weight matrix W on thetransmitting side 1 in advance, and the transmission beam measure (firstlevel measuring section) 24 measures a level for each beam from thetransmitter 1 based on the pilot replica stored in the known pilotmemory 25 and the information of the transmission weight matrix W storedin the known transmission weight memory 26.

The known selectable beam memory 28 stores information regardingselectable beam in advance, and in this embodiment, as shown in FIG. 3for example, a comparative beam ID table 281 in which the number oftransmission streams and a candidate beam ID are related to each otheris stored. Meanwhile, in the comparative beam ID table 281 shown in FIG.3, it is illustrated that in a case that the number of transmissionstreams is four, the candidate of the selectable beam (candidate beam)is one beam, ID=0, in a case that the number of transmission streams istwo, the IDs of the candidate beam are 2, 4, 6, 8 (or 1, 3, 5, 7), thatis to say, nonadjacent four beams of even number (or odd number) IDs,and in a case that the number of transmission streams is one, the IDs ofthe candidate beam are 9 beams, 1 to 9.

The transmission beam ID/stream determiner (first determining section)27 determines information regarding the transmission beam ID (the numberof transmission beams) and the number of transmission streams (beamselection information) to be transmitted to the transmitter 1 as thefeedback information, based on a measurement result by the transmissionbeam measure 24 and the information (comparative beam ID table 281)stored in the known selectable beam memory 28, and in this example, asshown in FIG. 3 for example, this is provided with a ranking levelcomparator 271, and by checking possibility in decreasing order of thenumber of transmission streams based on a measurement level (Level[ID])of each beam (ID) measured by the transmission beam measure 24, athreshold (TH[k]) corresponding to the number of transmission streams(k) and contents of the comparative beam ID table 281, in the rankinglevel comparator 271, the number of transmission streams and the beam IDat that time are determined. Meanwhile, in FIG. 3, a case in which themaximum number of transmission streams is four (that is to say, k=1 to4) is shown.

That is to say, the block 20 including the above-described transmissionbeam measure 24, the known pilot memory 25, the known transmissionweight memory 26, the transmission beam ID/stream determiner 27 and theknown selectable beam memory 28 functions as beam selective receptioncontrol means for selectively receiving any one or more of thetransmission beams through the receiving antennas 21-j from thetransmission beams of which number of transmission beams formed fortransmitting the transmission stream is controlled depending on thenumber of transmission streams by the transmitter 1.

Meanwhile, the information determined by the transmission beam ID/streamdeterminer 27 is fed back (reported) to the transmitter 1 through atransmission system of the receiver 2 not shown, as control informationfor transmission beam control (the number of beams to be formed) by thebeam controller 15 in the transmitter 1. Therefore, the transmitter 1(beam controller 15) operates according to the above-described controlinformation (feedback information), thereby controlling the beamselector 13 and the multibeam former 14 to increase the number oftransmission beams in proportion as the number of transmission streamsis smaller, and controlling the beam selector 13 and the multibeamformer 14 to perform the element transmission by one beam of beam ID=0when the number of transmission streams is the maximum value.

Hereinafter, an operation (beam selection method) of the MIMOtransmission system of this embodiment configured as above is describedin detail.

First, the transmitter 1 uses a constantly uniform fixed weight as thetransmission weight (matrix) W of the multibeam, and multiplexes theorthogonal pilot signal pi for each of the transmitting antennas 16-i totransmit. That is to say, in the user selector 11, under the control ofthe scheduler 15, one or more user data stream to be transmitted isselected from a plurality of series of user data streams and is input tothe channel coder/modulator 12, and in the channel coder/modulator 12,under the control of the scheduler 15, the required error correctioncoding such as the turbo coding is performed with the specified codingratio, and after that, the obtained bit series is mapped to the symbolhaving the signal point (signal of the data channel) such as thespecified modulating scheme (QPSK or 16QAM) and is modulated.

The obtained modulated data is input to the beam selector 13, and in thebeam selector 13, under the control of the scheduler 15, the beam usedfor transmitting the modulated data is selected by the number dependingon the number of streams to be transmitted from a plurality of fixedbeams (multibeam) formed by the multibeam former 14, and the modulateddata is transmitted from the transmitting antennas 16 by the selectedbeam. On this occasion, the orthogonal pilot signal pi is multiplexed byeach adder 17-i of the element pilot multiplex section 17 and istransmitted from each transmitting antenna 16-i.

On the other hand, in the receiver 2, the signal transmitted from theabove-described transmitter 1 by the multibeam is received by eachreceiving antenna 21-j and is input to the MIMO/SIMO demodulator 22 andthe transmission beam measure 24, respectively. In the MIMO/SIMOdemodulator 22, the received signal from each receiving antenna 21-j isMIMO demodulated or SIMO demodulated to generate the user data stream.That is to say, the user data stream is separated based on the channelestimate value (channel matrix) to generate the demodulated data.

An error correction decoding such as a turbo decoding is performed tothe obtained demodulated data by the decoder 23, thereby, decoded dataof the user data stream may be obtained.

On the other hand, in the transmission beam measure 24, a level for eachbeam is measured based on the pilot replica in the known pilot memory 25and the information of the known transmission weight W in the knowntransmission weight memory 26 (hereinafter, also referred to as atransmission weight information W).

For example, when the transmission data vector, the transmission weightinformation (matrix), pilot vector and channel information (matrix) arerepresented as X=[x1, . . . , xn], W=[W1, . . . , Wm] (wherein, mrepresents the number of transmission beams), P=[p1, . . . , pn] andH=[H1, . . . , Hn], respectively, and the received signal on thereceiving side 2 is represented as Y, Y=HP=HWX is received on thereceiving side 2.

Therefore, in the transmission beam measure 24, by obtaining the channelinformation H of each element (transmitting antenna 16-i) by using theknown pilot vector P, and by obtaining HW by using the knowntransmission weight information W, it becomes possible to obtain thechannel information of each transmission beam, so that the levelmeasurement for each beam becomes possible based on the channelinformation.

Then, the level measurement result (Level [ID]) for each obtained beam(ID) is input to the transmission beam ID/stream determiner 27, and bychecking the possibility in the descending order of the number of thetransmission streams by the ranking level comparator 271 based on thelevel measurement result, the threshold (TH[k]) corresponding to thenumber of transmission streams (k) and the contents of the comparativebeam ID table 281, the number of transmission streams and the beam ID atthat time are determined.

That is to say, as shown in FIG. 4 for example, in a case in which themaximum number of transmission streams is four (beam ID=0), the rankinglevel comparator 271 first compares the level measurement resultLevel[ID=0] of the beam ID=0 and the threshold TH[k=4] corresponding tothe number of transmission streams k=4 to check whether an equationLevel[ID=0]>TH[k=4] is satisfied or not (possibility that the number oftransmission streams is four) (step S1). As a result, when the equationLevel[ID=0]>TH[k=4] is satisfied, the ranking level compator 271determines that the number of transmission streams k=4 and the beam ID=0(route Y of step 1 to step S2).

On the other hand, when the equation Level[ID=0]≦TH[k=4] is satisfied,the ranking level comparator 271 selects two IDs (IDmax1, IDmax2) ofwhich levels are larger from the level measurement results Level[ID=1],Level[ID=2], Level[ID=3] and Level[ID=4] (route N of step S1 to stepS3), and compares each level measurement result Level[ID=IDmax1],Level[ID=IDmax2] and the threshold TH[k=2] corresponding to the numberof transmission streams k=2 to check whether both of the levelmeasurement results Level[ID=IDmax1], Level[ID=IDmax2] are larger thanthe threshold TH[k=2] or not (possibility of that the number oftransmission streams is two) (step S4).

As a result, if both of the level measurement results Level[ID=IDmax1],Level[ID=IDmax2] are larger than the threshold TH[k=2], the rankinglevel comparator 271 determines that the number of transmission streamsk=2 and the beam ID=IDmax2, IDmax2 (route Y of step S4 to step S5).

On the other hand, if one or both of the level measurement resultsLevel[ID=IDmax1], Level[ID=IDmax2] is not larger than the thresholdTH[k=2], the ranking level comparator 271 selects the maximum ID fromthe level measurement results Level[ID=1] to Level[ID9] as the IDmax(route N of step S4 to step S6), and checks whether the levelmeasurement result Level[IDmax] is larger than the threshold TH[1]corresponding to the number of transmission streams k=1 or not(possibility of that the number of transmission streams is one) (stepS7).

As a result, if the level measurement result Level[IDmax] is larger thanthe threshold TH[1], the ranking level comparator 271 determines thatthe number of transmission streams k=1 and the beamID=IDmax (route Y ofstep S7 to step S8), otherwise, determines that the number oftransmission streams and the beam ID are not allocated (route N of stepS7 to step S9).

As described above, in the transmission beam ID/stream determiner 27,the beam ID having the maximum channel information reliability isselected from the predetermined known selectable beams depending on thenumber of streams requesting the transmission.

The specific example is described by using an image diagram shown inFIG. 5. FIG. 5 illustrates a case in which the transmitter 1, the numberof transmitting antennas of which is n=4, may multibeam transmit withthe maximum number of transmission beams is nine (beam IDs=1, 2, . . . ,9) or element transmit (ID=0) the number of transmission streams k=1 to4.

The receiver 2 selects one beam of which reception level (receptionquality) is the maximum out of entire nine beams 30-1 to 30-9 (beamIDs=1 to 9) by the above described algorithm when the number oftransmission streams k=1, and selects two beams of which receptionlevels are high out of four beams 30-2, 30-4, 30-6, 30-8 of the evennumber beam IDs (IDs=2, 4, 6, 8) [or from five beams 30-1, 30-3, 30-5,30-7, 30-9 of the odd number beam IDs (IDs=1, 3, 5, 7, 9)] as the knownselectable beams when the number of transmission streams k=2. That is tosay, this selects the beam under the limited condition that theinterbeam correlation therebetween is low (not adjacent). Then, when thenumber of transmission streams increases to k=4, the receiver 2 does notselect the beam under the similarly limited conditions or ultimately (ina case of the maximum number of transmission streams), this does notselects beam and receives one beam 30-0 by the element transmission(beam ID=0).

In this manner, when the number of transmission streams is small, thebeam is selected from a number of beams of which beam directions aredifferent to each other. Specifically, when the number of transmissionstreams k is not smaller than 2 (MIMO transmission), the beam isselected from orthogonal multibeam (or multibeam equivalent thereto) asthe multibeam, when the number of transmission streams k is the maximum,the beam is not selected and one beam of the element transmission isdirectly received, and when the number of transmission streams k is theminimum (k=1) (SIMO transmission), the beam is selected from more beamsarranged so as not to reduce gain due to the direction thereof by addingthe beams faced the direction to compensate between the beams to theselectable candidate beam relative to the orthogonal beam (or the beamequivalent thereto) as the multibeam.

Then, the information of the number of transmission streams k and thebeam ID determined by the receiver 2 (transmission beam ID/streamdeterminer 27) as described above is sent back to the transmitter 1through the transmission system of the receiver 2 not shown as thefeedback information. That is to say, in this example, the transmissionbeam ID/stream determiner 27 also functions as a first report sectionfor reporting the information regarding the determined number oftransmission streams and the transmission beams as the controlinformation of the number of transmission beams formed in thetransmitter 1.

On the transmitting side 1, the above-described feedback informationfrom the receiving side 2 is reported to the beam controller 15 througha reception system of the transmitter 1 not shown, and the beamcontroller 15 controls the user selector 11, the channel coder/modulator12 and the beam selector 13 based on the feedback information, andselects the number of transmission streams and the beam to perform thebeam control of the transmission stream.

As described above, according to this embodiment, by controlling(changing) the number of candidate beams (the number of transmissionbeams to be formed) selectable on the receiving side 2 depending on thenumber of transmission streams on the transmitting side 1, for example,it becomes possible to obtain the gain as large as possible by theelement transmission at the time of one stream and to perform the beamselection such that the interbeam correlation therebetween becomes lowat the time of multistream, and the excellent throughput characteristics(reception characteristics) may be obtained while attaining both highthroughput characteristics due to the low interbeam correlation andhighly directional gain, without increasing the feedback informationamount to the transmitter 1.

[C] Description of Second Embodiment

FIG. 6 is a block diagram illustrating a configuration of the MIMOtransmission system according to a second embodiment. Although the MIMOtransmission system shown in FIG. 6 also is provided with the MIMOtransmitter 1 and the MIMO receiver 2, this is different from theabove-described configuration in FIG. 2 in that in the MIMO transmitter1, a beam pilot multiplexer 17 a is provided on a previous stage of themultibeam former 14 (subsequent stage of the beam selector 13) in placeof the element pilot multiplexer 17, and in the MIMO receiver 2, theknown transmission weight memory 26 is not required in the block 20,which functions as the beam selective reception control means.Meanwhile, other components designated by the same reference numerals asalready described ones are same as or similar to the already describedcomponents unless otherwise noted. In addition, in this example also,the transmission weight information W=[W1, . . . , Wm] in the multibeamformer 14 is fixed as in the first embodiment.

Herein, the beam pilot multiplexer (second pilot multiplex means) 17 ais provided with adders (multiplex circuits) 17 a-1 to 17 a-mcorresponding to each output depending on the number of transmissionbeams (maximum m) of the beam selector 13 and is for multiplexingorthogonal pilot signals p1 to pm for each beam of the multibeam by theadders 17 a-1 to 17 a-m.

Therefore, on the receiving side 2 (transmission beam measure 24), evenif the transmission weight information W on the transmitting side 1 isnot known (if the already described known transmission weightinformation memory 26 is not provided), the channel information of eachtransmission beam may be estimated based on the known pilot replica inthe known pilot memory 25, so that the level measurement for each beamas in the first embodiment becomes possible. That is to say, thetransmission beam measure 24 of this example functions as a second levelmeasuring section for measuring level of the transmission beam based onthe above-described pilot signal (replica).

Therefore, in this example also, in the transmission beam ID/streamdeterminer 27 (ranking level comparator 271), it is possible to performthe beam selection (determination of the beam ID and the number oftransmission streams) depending on the number of transmission streamsbased on the information in the known selectable beam memory 28(comparative beam ID table 281) as in the above-described algorithm inFIG. 4 (steps S1 to S9) and feed back the information to thetransmitting side 1.

That is to say, the transmission beam ID/stream determiner 27 of thisexample functions as a second determining section for determining thenumber of transmission streams and the transmission beams to be receivedbased on the level measuring result by the transmission beam measure 24as the above-described second level measuring section, and alsofunctions as a second report section for reporting the informationregarding the number of transmission streams and the transmission beams,which are determined, as the control information of the number oftransmission beams in the transmitter 1.

Then, in this case, the beam controller 15 of the transmitter 1 alsofunctions as second reported information receiving means for receivingthe information regarding the number of transmission streams and thetransmission beam, determined based on the level measurement resultmeasured for the transmission beam based on the pilot signal asdescribed above in the receiver 2 and is reported from the receiver 2,and performs the control of the number of transmission beams based onthe received information.

Therefore, the effect and advantage similar with those of the firstembodiment may be obtained, and in this example, the known transmissionweight memory 26 is not required on the receiving side 2, so that it ispossible to simplify the configuration and process on the receiving side2.

[D] Description of Third Embodiment

FIG. 7 is a block diagram illustrating a configuration of the MIMOtransmission system according to a third embodiment, and although theMIMO transmission system shown in FIG. 7 also is provided with the MIMOtransmitter 1 and the MIMO receiver 2, this is different from theconfiguration described above in FIG. 2 in that in the MIMO transmitter1, a broadcast information adder 18 is provided on a subsequent stage ofthe multibeam former 14 and a weight generator 19 a and a selectablebeam information generator 19 b are provided, and in the MIMO receiver2, in the block 20, which functions as the beam selective receptioncontrol means, a broadcast information extractor 29 is provided and theknown transmission weight memory 26 and the known selectable beam memory28 (comparative beam ID table 281) are not required. Meanwhile, othercomponents indicated by the same reference numerals as the alreadydescribed ones are the same as or similar to the already describedcomponents unless otherwise noted.

Here, in the transmitter 1, the weight generator 19 a adaptivelygenerates the transmission weight information W=[W1, . . . , Wm] used inthe multibeam former 14. That is to say, the beam former 14 of thisexample functions as a second beam former for performing the beamtransmission by the variable transmission weight information W.

The selectable beam information generator 19 b generates the informationregarding the beam selectable on the receiving side 2 (limitingcondition of beam selection), for example, information corresponding tothe contents of the comparative beam ID table 281 described above inFIG. 3.

That is to say, in this example, it is possible to change (control) thetransmission weight information W, and the information regarding thebeam selectable by the transmission weight information W and the numberof transmission streams (hereinafter, referred to as selectable beaminformation).

The broadcast information adder (broadcasting means) 18 is required toreport the variable information to the receiving side 2, so that this isfor adding (multiplexing) to the transmission stream as the informationsuch as a (downlink) inform channel to the receiving side 2. Meanwhile,an updating period of the broadcast information, that is to say, theupdating period of the transmission weight information W by the weightgenerator 19 a and the updating period of the selectable beaminformation by the selectable beam information generator 19 b are setdepending on the system.

On the other hand, in the receiver 2, the broadcast informationextractor section 29 is for extracting the broadcast information(transmission weight information W and the selectable beam information)from the signal demodulated by the MIMO/SIMO demodulator 22, and thetransmission weight information W and the selectable beam informationout of the extracted broadcast information are configured to be sent tothe transmission beam measuring section 24 and the transmission beamID/stream determiner 27, respectively.

Therefore, in the transmission beam measure 24 of this example, thechannel estimation of each beam becomes possible based on thetransmission weight information W broadcasted from the transmitter 1 andextracted by the broadcast information extractor 29 and the pilotreplica in the known pilot memory 25, and the level measurement for eachbeam as in the first embodiment becomes possible without requiring thealready described known transmission weight memory 26. That is to say,the transmission beam measure 24 of this example functions as a thirdlevel measuring section for performing the level measurement of thetransmission beam based on the pilot signal and the transmission weightinformation W broadcasted from the transmitter 1.

Also, the transmission beam ID/stream determiner 27 may perform the beamselection (determination of the beam ID and the number of transmissionstreams) depending on the number of transmission streams as in theabove-described algorithm in FIG. 4 (steps S1 to S9) based on theselectable beam information extracted by the broadcast informationextractor 29, the level measurement result by the transmission beammeasure 24 and the threshold (TH[k]) depending on the above-describednumber of transmission streams, and feedback the information to thetransmitting side 1. This means that in the receiver 2, the informationcorresponding to the above-described comparative beam ID table 281 maybe built or updated based on the information extracted by the broadcastinformation extractor 29.

That is to say, the transmission beam ID/stream determiner 27 of thisexample functions as a third determining section operable to determinethe number of transmission streams and the transmission beam to bereceived based on the level measurement result by the transmission beammeasure 24 as the third level measuring section and the selectable beaminformation broadcasted from the transmitter 1 (information regardingthe number of transmission beams), and also functions as a third reportsection to report the information regarding the number of transmissionstreams and the transmission beam, which are determined, as the controlinformation of the number of transmission beams in the transmitter 1.

Then, in the transmitter 1, the above-described control information(feedback information) reported from the receiver 2 is received at thebeam controller 15, and based on the information, the transmission beamcontrol is performed. That is to say, the beam controller 15 of thisexample also functions as third reported information receiving means toreceive the information regarding the number of transmission streams andthe transmission beam, determined based on the level measurement resultmeasured for the transmission beam based on the pilot signal and thetransmission weight information W broadcasted by the broadcastinformation adder 18 and the selectable beam information broadcasted bythe broadcast information adder 18, and is reported from the receiver 2,and controls the number of transmission beams based on the receivedinformation, in the receiver 2, as described above.

Therefore, the effect and advantage same as or similar to those in thefirst embodiment may be obtained, and in this example, by adaptivelychanging the transmission weight information W and the selectable beaminformation depending on a communication environment between thetransmitter 1 and the receiver 2, it is possible to achieve the optimalbeam selection depending on the communication environment, therebyfurther improving the throughput characteristics.

Meanwhile, although both of the transmission weight information W andthe selectable beam information are made variable and each informationis broadcasted to the receiving side 2 in the above-describedembodiment, it is possible that only one of them is made variable and isbroadcasted to the receiving side 2.

In addition, it is possible to multiplex the orthogonal pilot signal foreach beam in this example also, as in the second embodiment. In thiscase, as described above, it is not required to know the transmissionweight information W on the receiving side 2, it is only necessary tobroadcast only the selectable beam information to the receiving side 2.

Meanwhile, it goes without saying that the present invention is notlimited to the above-described embodiments and various changes may bemade without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

As described above in detail, according to the embodiments, by changingthe number of transmission beams (the original number of beamsselectable on the receiving side) to be formed depending on the numberof transmission streams, it is possible to achieve the system performingthe excellent communication, that is to say, the high throughputcharacteristics by low interbeam correlation at the time of themultistream transmission and the large directional gain at the time ofthe single stream, by the small feedback information only for the beamselection, so that this is considered to be extremely useful in thefield of wireless communication technique.

What is claimed is:
 1. A wireless transmission method capable oftransmitting a data stream by a wireless transmitter having a pluralityof transmitting antennas, the wireless transmission method comprising:controlling the number of candidates of selectable transmission beams tobe formed for transmitting the data stream so that said number ofcandidates of selectable transmission beams increases as the number ofdata streams becomes smaller, said transmission beams are formed bypredetermined number of the plurality of transmitting antennas.
 2. Thewireless transmission method according to claim 1, further comprising:receiving said data streams by a wireless receiver.
 3. The wirelesstransmission method according to claim 1, wherein a wireless receiverreceives two or more of said transmission beams having a low correlationtherebetween, upon said number of data streams being two or larger. 4.The wireless transmission method according to claim 3, wherein saidwireless receiver receives nonadjacent transmission beams as saidtransmission beams having a low correlation therebetween.
 5. Thewireless transmission method according to claim 1, wherein a wirelesstransmitter multiplexes a pilot signal for each of said transmittingantennas to perform beam transmission by a fixed weighting coefficient,said wireless receiver measures a level of said transmission beam basedon said pilot signal and said fixed weighting coefficient, determinessaid number of data streams and said transmission beam to be receivedbased on the measured level, and reports information regarding saidnumber of data streams and said transmission beam, which are determined,to said wireless transmitter, and said wireless transmitter controlssaid number of candidates of selectable transmission beams based on theinformation reported from said wireless receiver.
 6. The wirelesstransmission method according to claim 1, wherein said wirelesstransmitter multiplexes a pilot signal for each of said transmissionbeams to perform beam transmission by a fixed weighting coefficient,said wireless receiver measures a level of said transmission beam basedon said pilot signal, determines said number of data streams and saidtransmission beam to be received based on the measured level, andreports information regarding said number of data streams and saidtransmission beam, which are determined, to said wireless transmitter,and said wireless transmitter controls said number of candidates ofselectable transmission beams based on the information reported fromsaid wireless receiver.
 7. The wireless transmission method according toclaim 1, wherein said wireless transmitter multiplexes a pilot signalfor each of said transmitting antennas to perform beam transmission by avariable weighting coefficient, and broadcasts information regardingsaid variable weighting coefficient and information regarding saidnumber of candidates of selectable transmission beams to said wirelessreceiver, a wireless receiver measures a level of said transmission beambased on said pilot signal and the information regarding said variableweighting coefficient broadcasted from said wireless transmitter,determines said number of data streams and said transmission beam to bereceived based on the measured level and the information regarding saidnumber of candidates of selectable transmission beams broadcasted fromsaid wireless transmitter, and reports information regarding said numberof data streams and said transmission beam, which are determined, tosaid wireless transmitter, and said wireless transmitter controls saidnumber of candidates of selectable transmission beams based on theinformation reported from said wireless receiver.
 8. A wirelesstransmitter capable of transmitting a data stream to a wirelessreceiver, the wireless transmitter comprising: a plurality oftransmitting antennas; and a controller configured to control the numberof candidates of selectable transmission beams to be formed fortransmitting said data stream so that said number of candidates ofselectable transmission beams increases as the number of data streamsbecomes smaller, said transmission beams are formed by predeterminednumber of the plurality of transmitting antennas.
 9. A wireless systemincluding the wireless transmitter according to claim 8, furthercomprising: said wireless receiver comprising a receiver configured toreceive said data streams.
 10. The wireless transmitter according toclaim 8, further comprising: a first multiplexer operable to multiplex apilot signal for each of said transmitting antennas; a first beam formeroperable to perform beam transmission by a fixed weighting coefficient;and a receiver that receives information regarding said number of datastreams and the transmission beam, determined based on a levelmeasurement result measured for said transmission beam based on saidpilot signal and said fixed weighting coefficient in said wirelessreceiver and reported from said wireless receiver, wherein saidcontroller controls said number of candidates of selectable transmissionbeams based on the information received by said receiver.
 11. Thewireless transmitter according to claim 8, further comprising: a secondmultiplexer operable to multiplex a pilot signal for each of saidtransmission beams; a first beam former operable to perform beamtransmission by a fixed weighting coefficient; and a receiver thatreceives information regarding said number of data streams and thetransmission beam, determined based on a level measurement resultmeasured for said transmission beam based on said pilot signal in saidwireless receiver and reported from said wireless receiver, wherein saidcontroller controls said number of candidates of selectable transmissionbeams based on the information received by said receiver.
 12. Thewireless transmitter according to claim 8, further comprising: a firstmultiplexer operable to multiplex a pilot signal for each of saidtransmitting antennas; a second beam former operable to perform beamtransmission by a variable weighting coefficient; a transmitter operableto broadcast information regarding said variable weighting coefficientand information regarding said number of candidates of selectabletransmission beams to said wireless receiver; and a receiver thatreceives information regarding said number of data streams andtransmission beam, determined based on a level measurement resultmeasured for said transmission beam based on said pilot signal and theinformation regarding said variable weighting coefficient broadcasted bysaid transmitter and the information regarding said number of candidatesof selectable transmission beams broadcasted by said transmitter, andreported from said wireless receiver, wherein said controller controlssaid number of candidates of selectable transmission beams based on theinformation received by said receiver.
 13. A wireless receiver capableof receiving a data stream from a wireless transmitter having aplurality of transmitting antennas by a multibeam, the wireless receivercomprising: a plurality of receiving antennas; and a receiver thatreceives data streams through said receiving antennas, wherein thenumber of candidates of selectable transmission beams to be formed fortransmitting said data stream increases as the number of said datastreams to be transmitted by said wireless transmitter becomes smaller,and said transmission beams are formed by predetermined number of theplurality of transmitting antennas.
 14. The wireless receiver accordingto claim 13, wherein said receiver receives two or more of saidtransmission beams having a low correlation therebetween, when saidnumber of data streams is two or larger.
 15. The wireless receiveraccording to claim 14, wherein said receiver receives nonadjacenttransmission beams as said transmission beams having a low correlationtherebetween.
 16. The wireless receiver according to claim 13, whereinsaid wireless transmitter multiplexes a pilot signal for each of saidtransmitting antennas to perform beam transmission by a fixed weightingcoefficient, and said wireless receiver further comprises: a controllerthat measures a level of said transmission beam based on said pilotsignal and said fixed weighting coefficient, determines said number ofdata streams and said transmission beam to be received based on themeasured level, and reports information regarding said determined numberof data streams and said determined transmission beam as controlinformation of said number of candidates of selectable transmissionbeams in said wireless transmitter.
 17. The wireless receiver accordingto claim 13, wherein said wireless transmitter multiplexes a pilotsignal for each of said transmitting beams to perform beam transmissionby a fixed weighting coefficient, and said wireless receiver furthercomprises: a controller that measures a level of said transmission beambased on said pilot signal, determines said number of data streams andsaid transmission beam to be received based on the measured level, andreports information regarding said determined number of data streams andsaid determined transmission beam as control information of said numberof candidates of selectable transmission beams in said wirelesstransmitter.
 18. The wireless receiver according to claim 13, whereinsaid wireless transmitter multiplexes a pilot signal for each of saidtransmitting antennas to perform beam transmission by a variableweighting coefficient, and broadcasts the information regarding saidvariable weighting coefficient and the information regarding said numberof candidates of selectable transmission beams to said wirelessreceiver, and said wireless receiver further comprises: a controllerthat measures a level of said transmission beam based on said pilotsignal and the information regarding said variable weighting coefficientbroadcasted from said wireless transmitter, determines said number ofdata streams and said transmission beam to be received based on themeasured level and the information regarding said number of candidatesof selectable transmission beams broadcasted from said wirelesstransmitter, and reports information regarding said determined number ofdata streams and said determined transmission beam as controlinformation of said number of candidates of selectable transmissionbeams in said wireless transmitter.